SSHFP: Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
Publish a fingerprint of an SSH server's key in DNS, signed with DNSSEC, so clients can check it instead of trusting blindly.
Schlyter, J. and W. Griffin, "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints", RFC 4255, DOI 10.17487/RFC4255, January 2006, <https://www.rfc-editor.org/info/rfc4255>.
@misc{rfc4255,
series = {Request for Comments},
number = 4255,
howpublished = {RFC 4255},
publisher = {RFC Editor},
doi = {10.17487/RFC4255},
url = {https://www.rfc-editor.org/info/rfc4255},
author = {J. Schlyter and W. Griffin},
title = {{Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints}},
pagetotal = 9,
year = 2006,
month = jan,
} RFC text
Contents
Network Working Group J. Schlyter
Request for Comments: 4255 OpenSSH
Category: Standards Track W. Griffin
SPARTA
January 2006
Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes a method of verifying Secure Shell (SSH) host
keys using Domain Name System Security (DNSSEC). The document
defines a new DNS resource record that contains a standard SSH key
fingerprint.
Table of Contents
1. Introduction ....................................................2
2. SSH Host Key Verification .......................................2
2.1. Method .....................................................2
2.2. Implementation Notes .......................................2
2.3. Fingerprint Matching .......................................3
2.4. Authentication .............................................3
3. The SSHFP Resource Record .......................................3
3.1. The SSHFP RDATA Format .....................................4
3.1.1. Algorithm Number Specification ......................4
3.1.2. Fingerprint Type Specification ......................4
3.1.3. Fingerprint .........................................5
3.2. Presentation Format of the SSHFP RR ........................5
4. Security Considerations .........................................5
5. IANA Considerations .............................................6
6. Normative References ............................................7
7. Informational References ........................................7
8. Acknowledgements ................................................8
1. Introduction
The SSH [6] protocol provides secure remote login and other secure
network services over an insecure network. The security of the
connection relies on the server authenticating itself to the client
as well as the user authenticating itself to the server.
If a connection is established to a server whose public key is not
already known to the client, a fingerprint of the key is presented to
the user for verification. If the user decides that the fingerprint
is correct and accepts the key, the key is saved locally and used for
verification for all following connections. While some security-
conscious users verify the fingerprint out-of-band before accepting
the key, many users blindly accept the presented key.
The method described here can provide out-of-band verification by
looking up a fingerprint of the server public key in the DNS [1][2]
and using DNSSEC [5] to verify the lookup.
In order to distribute the fingerprint using DNS, this document
defines a new DNS resource record, "SSHFP", to carry the fingerprint.
Basic understanding of the DNS system [1][2] and the DNS security
extensions [5] is assumed by this document.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [3].
2. SSH Host Key Verification
2.1. Method
Upon connection to an SSH server, the SSH client MAY look up the
SSHFP resource record(s) for the host it is connecting to. If the
algorithm and fingerprint of the key received from the SSH server
match the algorithm and fingerprint of one of the SSHFP resource
record(s) returned from DNS, the client MAY accept the identity of
the server.
2.2. Implementation Notes
Client implementors SHOULD provide a configurable policy used to
select the order of methods used to verify a host key. This document
defines one method: Fingerprint storage in DNS. Another method
defined in the SSH Architecture [6] uses local files to store keys
for comparison. Other methods that could be defined in the future
might include storing fingerprints in LDAP or other databases. A
configurable policy will allow administrators to determine which
methods they want to use and in what order the methods should be
prioritized. This will allow administrators to determine how much
trust they want to place in the different methods.
One specific scenario for having a configurable policy is where
clients do not use fully qualified host names to connect to servers.
In this scenario, the implementation SHOULD verify the host key
against a local database before verifying the key via the fingerprint
returned from DNS. This would help prevent an attacker from
injecting a DNS search path into the local resolver and forcing the
client to connect to a different host.
2.3. Fingerprint Matching
The public key and the SSHFP resource record are matched together by
comparing algorithm number and fingerprint.
The public key algorithm and the SSHFP algorithm number MUST
match.
A message digest of the public key, using the message digest
algorithm specified in the SSHFP fingerprint type, MUST match the
SSHFP fingerprint.
2.4. Authentication
A public key verified using this method MUST NOT be trusted if the
SSHFP resource record (RR) used for verification was not
authenticated by a trusted SIG RR.
Clients that do validate the DNSSEC signatures themselves SHOULD use
standard DNSSEC validation procedures.
Clients that do not validate the DNSSEC signatures themselves MUST
use a secure transport (e.g., TSIG [9], SIG(0) [10], or IPsec [8])
between themselves and the entity performing the signature
validation.
3. The SSHFP Resource Record
The SSHFP resource record (RR) is used to store a fingerprint of an
SSH public host key that is associated with a Domain Name System
(DNS) name.
The RR type code for the SSHFP RR is 44.
3.1. The SSHFP RDATA Format
The RDATA for a SSHFP RR consists of an algorithm number, fingerprint
type and the fingerprint of the public host key.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| algorithm | fp type | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
/ /
/ fingerprint /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.1.1. Algorithm Number Specification
This algorithm number octet describes the algorithm of the public
key. The following values are assigned:
Value Algorithm name
----- --------------
0 reserved
1 RSA
2 DSS
Reserving other types requires IETF consensus [4].
3.1.2. Fingerprint Type Specification
The fingerprint type octet describes the message-digest algorithm
used to calculate the fingerprint of the public key. The following
values are assigned:
Value Fingerprint type
----- ----------------
0 reserved
1 SHA-1
Reserving other types requires IETF consensus [4].
For interoperability reasons, as few fingerprint types as possible
should be reserved. The only reason to reserve additional types is
to increase security.
3.1.3. Fingerprint
The fingerprint is calculated over the public key blob as described
in [7].
The message-digest algorithm is presumed to produce an opaque octet
string output, which is placed as-is in the RDATA fingerprint field.
3.2. Presentation Format of the SSHFP RR
The RDATA of the presentation format of the SSHFP resource record
consists of two numbers (algorithm and fingerprint type) followed by
the fingerprint itself, presented in hex, e.g.:
host.example. SSHFP 2 1 123456789abcdef67890123456789abcdef67890
The use of mnemonics instead of numbers is not allowed.
4. Security Considerations
Currently, the amount of trust a user can realistically place in a
server key is proportional to the amount of attention paid to
verifying that the public key presented actually corresponds to the
private key of the server. If a user accepts a key without verifying
the fingerprint with something learned through a secured channel, the
connection is vulnerable to a man-in-the-middle attack.
The overall security of using SSHFP for SSH host key verification is
dependent on the security policies of the SSH host administrator and
DNS zone administrator (in transferring the fingerprint), detailed
aspects of how verification is done in the SSH implementation, and in
the client's diligence in accessing the DNS in a secure manner.
One such aspect is in which order fingerprints are looked up (e.g.,
first checking local file and then SSHFP). We note that, in addition
to protecting the first-time transfer of host keys, SSHFP can
optionally be used for stronger host key protection.
If SSHFP is checked first, new SSH host keys may be distributed by
replacing the corresponding SSHFP in DNS.
If SSH host key verification can be configured to require SSHFP,
SSH host key revocation can be implemented by removing the
corresponding SSHFP from DNS.
As stated in Section 2.2, we recommend that SSH implementors provide
a policy mechanism to control the order of methods used for host key
verification. One specific scenario for having a configurable policy
is where clients use unqualified host names to connect to servers.
In this case, we recommend that SSH implementations check the host
key against a local database before verifying the key via the
fingerprint returned from DNS. This would help prevent an attacker
from injecting a DNS search path into the local resolver and forcing
the client to connect to a different host.
A different approach to solve the DNS search path issue would be for
clients to use a trusted DNS search path, i.e., one not acquired
through DHCP or other autoconfiguration mechanisms. Since there is
no way with current DNS lookup APIs to tell whether a search path is
from a trusted source, the entire client system would need to be
configured with this trusted DNS search path.
Another dependency is on the implementation of DNSSEC itself. As
stated in Section 2.4, we mandate the use of secure methods for
lookup and that SSHFP RRs are authenticated by trusted SIG RRs. This
is especially important if SSHFP is to be used as a basis for host
key rollover and/or revocation, as described above.
Since DNSSEC only protects the integrity of the host key fingerprint
after it is signed by the DNS zone administrator, the fingerprint
must be transferred securely from the SSH host administrator to the
DNS zone administrator. This could be done manually between the
administrators or automatically using secure DNS dynamic update [11]
between the SSH server and the nameserver. We note that this is no
different from other key enrollment situations, e.g., a client
sending a certificate request to a certificate authority for signing.
5. IANA Considerations
IANA has allocated the RR type code 44 for SSHFP from the standard RR
type space.
IANA has opened a new registry for the SSHFP RR type for public key
algorithms. The defined types are:
0 is reserved
1 is RSA
2 is DSA
Adding new reservations requires IETF consensus [4].
IANA has opened a new registry for the SSHFP RR type for fingerprint
types. The defined types are:
0 is reserved
1 is SHA-1
Adding new reservations requires IETF consensus [4].
6. Normative References
[1] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
[2] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[5] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033, March
2005.
Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Resource Records for the DNS Security Extensions", RFC 4034,
March 2005.
Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Protocol Modifications for the DNS Security Extensions", RFC
4035, March 2005.
[6] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[7] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, January 2006.
7. Informational References
[8] Thayer, R., Doraswamy, N., and R. Glenn, "IP Security Document
Roadmap", RFC 2411, November 1998.
[9] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, May 2000.
[10] Eastlake 3rd, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, September 2000.
[11] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.
8. Acknowledgements
The authors gratefully acknowledge, in no particular order, the
contributions of the following persons:
Martin Fredriksson
Olafur Gudmundsson
Edward Lewis
Bill Sommerfeld
Authors' Addresses
Jakob Schlyter
OpenSSH
812 23rd Avenue SE
Calgary, Alberta T2G 1N8
Canada
EMail: jakob@openssh.com
URI: http://www.openssh.com/
Wesley Griffin
SPARTA
7075 Samuel Morse Drive
Columbia, MD 21046
USA
EMail: wgriffin@sparta.com
URI: http://www.sparta.com/
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AI ELI5
On this page
§1The First-Connection Problem
An SSH client that connects to a server whose public key it does not know shows the user a fingerprint of the key. If the user accepts it, the key is saved locally and used to check every later connection. The RFC notes that some security-conscious users verify the fingerprint out-of-band, but many users blindly accept the key.
This document offers the out-of-band check: look up a fingerprint of the server's public key in the DNS and use DNSSEC to verify the lookup. To carry the fingerprint it defines a new DNS resource record, SSHFP. The text assumes you already understand DNS and the DNS security extensions.
The first time you meet someone online, you only have their word that they are who they say. Many people just say yes without checking. This memo gives you a safe way to check.
§2Verifying a Host Key with SSHFP
On connecting, the SSH client may look up the SSHFP records for the host. If the algorithm and fingerprint of the key the server presented match one of the returned records, the client may accept the identity of the server.
Matching has two parts. The public key algorithm and the SSHFP algorithm number MUST match, and a message digest of the public key, computed with the algorithm named by the SSHFP fingerprint type, MUST match the SSHFP fingerprint.
Client implementors SHOULD provide a configurable policy for the order of verification methods. DNS storage is one method, and local key files (from the SSH Architecture, RFC 4251) are another. If clients connect with names that are not fully qualified, the implementation SHOULD check the host key against a local database before using DNS. This helps stop an attacker who injects a DNS search path into the local resolver to push the client toward a different host.
The client asks the phone book what the server's fingerprint should be. If the fingerprint on the server's key is the same as the one in the phone book, it is a match. You can also decide which to check first, your own notes or the phone book.
§3Trusting the DNS Answer
A key verified this way MUST NOT be trusted if the SSHFP record was not authenticated by a trusted SIG record. A client that validates DNSSEC signatures itself SHOULD use standard DNSSEC validation procedures.
A client that does not validate signatures itself MUST use a secure transport between itself and whatever does the validation. The RFC gives TSIG, SIG(0) and IPsec as examples.
Without this step, the fingerprint in DNS would be just as easy to forge as the key it is meant to check.
A note in the phone book only helps if it has an unbroken seal. If the seal is missing or broken, the client must not believe the note.
§4The SSHFP Record
The SSHFP resource record stores a fingerprint of an SSH public host key tied to a DNS name. Its RR type code is 44.
The data part holds three things: an algorithm number (one octet), a fingerprint type (one octet) and the fingerprint itself. The algorithm number says which kind of public key it is: 0 is reserved, 1 is RSA and 2 is DSS. The fingerprint type names the message digest used: 0 is reserved and 1 is SHA-1.
The fingerprint is calculated over the public key blob as described in the SSH Transport Layer Protocol (RFC 4253). The digest output is placed as-is in the record. Reserving further values requires IETF consensus, and the RFC advises reserving as few fingerprint types as possible for interoperability, and only to increase security.
In text form, the record is the two numbers followed by the fingerprint in hex. Mnemonics instead of numbers are not allowed.
The record is a small card with three things on it: what kind of lock the server uses, how the fingerprint was made, and the fingerprint itself written in numbers and letters.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
ALGORITHM | Algorithm | 1 byte | Public key algorithm: 1 is RSA, 2 is DSS. | |
FP-TYPE | Fingerprint Type | 1 byte | Message digest used: 1 is SHA-1. | |
FINGERPRINT | Fingerprint (variable) | 2 bytes | Digest of the public key blob, placed as-is; length depends on the digest. |
§5Security Considerations
If a user accepts a key without checking the fingerprint through a secured channel, the connection is open to a man-in-the-middle attack. SSHFP improves this, but its overall security depends on the policies of the SSH host administrator and the DNS zone administrator, on how verification is done in the client, and on the client's diligence in using DNS securely.
Order matters. If SSHFP is checked first, new host keys can be distributed by replacing the SSHFP in DNS. If verification can be configured to require SSHFP, host key revocation can be done by removing the record from DNS.
DNSSEC only protects the fingerprint after the zone administrator signs it. So the fingerprint must reach the zone administrator securely, either by hand between the administrators or through secure DNS dynamic update between the SSH server and the name server. The RFC compares this to sending a certificate request to a certificate authority.
A different fix for the search-path problem is a trusted DNS search path, one not acquired through DHCP or other autoconfiguration. The RFC notes that current lookup APIs cannot tell whether a search path is trustworthy, so the whole client system would have to be configured that way.
The phone book is only as honest as the people who write in it and the way the note gets to them. The seal protects the note once it is in the book, but someone must still hand it over safely.
§6IANA Registrations
IANA allocated RR type code 44 for SSHFP from the standard RR type space. It also opened two new registries for the SSHFP record: one for public key algorithms (0 reserved, 1 RSA, 2 DSA) and one for fingerprint types (0 reserved, 1 SHA-1). Adding new reservations to either requires IETF consensus.
The keepers of the internet's lists gave this new kind of note its own number, 44, and started two small lists of allowed key kinds and fingerprint recipes.